This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-296363, filed on Sep. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to a printed board for transmitting signals. More particularly, the invention relates to a printed board for transmitting signals, the printed board which has a structure for practically equalizing propagation times of transmission lines connecting the same pair of circuit elements, a method for producing the printed board, and an electronic device that includes the printed board.
2. Description of the Related Art
Each high frequency transmission line, such as a CPU bus, for connecting a pair of circuit elements arranged on a printed board is prescribed with an allowable delay between a driver and a receiver. The delay includes a delay that occurs on the transmission line, namely, a wiring delay, as well as delays due to unique buffer characteristics of the driver and receiver.
Each wiring delay is prescribed with both a maximum value (maximum propagation time) and a minimum value (minimum propagation time) in consideration of characteristics of corresponding circuit elements. Accordingly, in conventional designing of printed boards, the maximum length and the minimum length of each transmission line between two circuit elements are determined by focusing on a propagation delay of each transmission line. The propagation delay is a value indicating how far a signal advances within a predetermined time.
For example, if two circuit elements are connected together at their terminals via two transmission lines, positions of a more distant pair of corresponding terminals are first determined, which provides the maximum transmission line length between the circuit elements. This determines a physical distance between these two circuit elements. Then positions of another pair of terminals are determined to provide the minimum transmission line length between the circuit elements. This may result in the transmission line meandering on the printed board. However, the meandering, or routing, requires an additional wiring space and causes unnecessary signal reflection.
A propagation delay Td of a transmission line is expressed by the following formula:
Td=(Lxc3x97{square root over ( )}∈r)/cxe2x80x83xe2x80x83(1)
The terms L, ∈r, and c respectively represent a length of the transmission line, a dielectric constant in the neighborhood of the transmission line, i.e., of the dielectric layer(s) through which the transmission line passes, and the velocity of light. Therefore the propagation delay can be adjusted by controlling the dielectric constant ∈r as well as the transmission line length L.
In consideration of such a condition, Japanese laid-open patent publication No. 11-8444 discloses a technique of equalizing propagation delays on every high frequency electrical wire. According to the publication, a dielectric layer is divided between electrical wires and a dielectric constant of each divided dielectric layer is changed according to a length of each electrical wire. The publication is based on the propagation delay Td of a signal on the electrical wire being proportional to the square root of the dielectric constant ∈r in the neighborhood of the electrical wire, as shown by formula (1).
As mentioned above, in the publication, dielectric constants of two insulators adjacent to each other in the same layer are different in order to adjust the propagation delay of each electrical wire. However, it is difficult to actually form a substrate having such a layer structure.
In accordance with the present invention, there is provided a printed board. The printed board comprises: a first dielectric layer having a dielectric constant larger than that of air, the layer having an upper surface and a lower surface; a first conducting layer formed under the lower surface of the first dielectric layer; a second dielectric layer, formed under the first conducting layer, having a dielectric constant larger than that of air; a second conducting layer formed under the second dielectric layer; a first circuit element having at least first and second terminals mounted on the upper surface of the first dielectric layer; a second circuit element, mounted on the upper surface of the first dielectric layer, having at least first and second terminals to be respectively paired with the first and second terminals of the first circuit element; a first transmission line, on the upper surface of the first dielectric layer, connecting the first terminals of the first and second circuit elements; and a second transmission line connecting the second terminals of the first and second circuit elements and being routed through the first dielectric layer, the first conducting layer, and the second dielectric layer, a distance between the second terminals of the first and second circuit elements being shorter than a distance between the first terminals of the first and second circuit elements.
Also in accordance with the present invention, there is provided a method for producing a printed board. The method for producing a printed board comprises: forming a first dielectric layer having a dielectric constant larger than that of air, the layer having an upper surface and a lower surface; forming a first conducting layer under the lower surface of the first dielectric layer; forming a second dielectric layer under the first conducting layer, the second dielectric layer having a dielectric constant larger than that of air; forming a second conducting layer under the second dielectric layer; mounting a first circuit element having at least first and second terminals on the upper surface of the first dielectric layer; mounting a second circuit element on the upper surface of the first dielectric layer, the second circuit element having at least first and second terminals to be respectively paired with the first and second terminals of the first circuit element; connecting the first terminals of the first and second circuit elements with a first transmission line on the upper surface of the first dielectric layer; and connecting the second terminals of the first and second circuit elements with a second transmission line routed through the first dielectric layer, the first conducting layer, and the second dielectric layer, a distance between the second terminals of the first and second circuit elements being shorter than a distance between the first terminals of the first and second circuit elements.
Further in accordance with the present invention, there is provided an electronic device. The electronic device comprises: a case; a first circuit element having at least first and second terminals; a second circuit element having at least first and second terminals to be respectively paired with the first and second terminals of the first circuit element; and a printed board mounted in the case. The printed board includes: a first dielectric layer having a dielectric constant larger than that of air, the first dielectric layer having an upper surface, on which the first circuit element and the second circuit element are mounted, and a lower surface; a first conducting layer formed under the lower surface of the first dielectric layer; a second dielectric layer, formed under the first conducting layer, having a dielectric constant larger than that of air; a second conducting layer formed under the second dielectric layer; a first transmission line, on the upper surface of the first dielectric layer, connecting the first terminals of the first and second circuit elements; and a second transmission line connecting the second terminals of the first and second circuit elements and being routed through the first dielectric layer, the first conducting layer, and the second dielectric layer, a distance between the second terminals of the first and second circuit elements being shorter than a distance between the first terminals of the first and second circuit elements.